A structural bifurcation of transport in toroidal plasmas

Plasma and Fusion Research

Nagashima

et al. 2018

As a result of the progresses in the study of plasma turbulence, the picture of 'multiple-scale turbulence' is widely accepted, and the 'nonlinear excitation, nonlocal interaction, probabilistic transition' views have been developed. Along the paths of these progresses, importance of new problem, i.e., the symmetry-breaking of turbulence structure (such as up-down asymmetry, excitation of streamer, etc.) is gradually recognized. In this topical review article, we revisit these issues, and illuminate the possibilities that the new progresses (which will be brought about by studying the symmetry-breaking of turbulence) are expected. Examining theoretical predictions and preceding experimental achievements, new advancements, which will be realized, are explained. The experimental study of symmetry-breaking of turbulence structure ultimately requires to measure fluctuations (at all scales) over the whole plasma cross-section simultaneously. In addition, new type of demands can be imposed in measuring of fluctuations over the whole plasma cross-section simultaneously. One of such demands is the accuracy of the measurement position. Problems in this aspect are also discussed. This concise review is used to identify what will be discovered and how it will be reached in future experiments, in the subject of the symmetry-breaking of turbulence structure.

Section: Large-scale Motion Of Turbulence Clumpsmentioning

confidence: 99%

“…The experimental study on the ballistic motion of turbulence and/or spontaneous corrugation of mean profile requires the simultaneous observation of microscopic fluctuations in a global scale. The preceding theories [97][98][99][100][101][102] have studied the dynamics and structure in radial direction.…”

Section: Large-scale Motion Of Turbulence Clumpsmentioning

confidence: 99%

Symmetry-Breaking of Turbulence Structure and Position Identification in Toroidal Plasmas

Plasma and Fusion Research

Nagashima

et al. 2018

“…We here briefly revisit theoretical formulas for experimental validation, which have been proposed based on the recent progress of diagnostics [20][21][22][23]. In this study, we use the model of normalized turbulence intensity, I/I 0 , in the presence of nonuniformity E r effects (as expressed by the parameter of Z) as follows:…”

Section: Theoretical Modelmentioning

confidence: 99%

“…As discussed in [20][21][22], the nonlinear decorrelation rate of fluctuations is enhanced by the shear of the radial electric field as…”

Section: Theoretical Modelmentioning

confidence: 99%

Reduction of ion thermal transport due to non-uniformity effects of the radial electric field at the edge of the JT-60U tokamak

Kamiya

2017

Nucl. Fusion

Nonuniformity effects of the edge radial electric field, E r , on edge transport barrier (ETB) formation have been identified with highspatiotemporal resolution spectroscopic measurement. We identified the decisive importance of E r curvature (i.e. 2nd derivative of E r ) on ETB formation corresponding to a local peak value in the pedestal region of the normalized ion temperature gradient, L −1 Ti ≡ −∇T i /T i . On the other hand, the E r shear (i.e. 1st derivative of E r ) was discovered to promote the expansion of pedestal width, compensating an unfavorable effect of the E r curvature having its sign dependence on the transport reduction/enhancement-predicted by a theoretical model as. A significant reduction of the inferred ion thermal transport coefficient in the pedestal region, χ i,ped. , due to the nonuniformity effects of E r was also confirmed quantitatively in the Hmode plasma at the initial ELMfree phase. When the inhomogeneity of radial electric field became strong enough, the inferred χ i,ped. value was close to its neoclassical one at the later ELMfree Hphase until 1st typeI ELM onset. Nevertheless, we observed the further reduction of electron density fluctuations (in the range up to 300 kHz) and D α signals in association with the increment of electric field inhomogeneity. These suggest that the electron turbulent transport is suppressed further.

“…Recently, a new model of criterion for turbulence suppression due to the non-uniformity of E r , including both its shear and curvature, has been proposed as follows [15];…”

Section: Introductionmentioning

confidence: 99%

Multiple plasma transport states in the H-mode transition on JT-60U

Kamiya

et al. 2019

Nucl. Fusion

Multiple turbulent plasma states in the edge transport barriers formation are studied on JT-60U. Following a slow L–H transition, which causes significant reduction in the ion thermal transport in the pedestal towards the neoclassical level with a weak negative Er value, we found clear and fast changes in the particle transport in association with the change in Er towards a strong negative value at the later H-phase. It has also been suggested that there is a clear difference in responses between ion and electron transports due to the change in the inhomogeneity in Er. This observation suggests the presence of multiple types of turbulent fluctuations in the H-mode plasma state, which affects the ion energy and other channels of transport differently. The nonlinear dynamics that induce the Er-bifurcation are also analyzed in terms of the radial current generation. These analyses contribute to understanding of the L–H transition condition.